This invention relates to paint or liquid spray gun apparatus, and specifically to a proximity sensor in combination with such a spray gun for purposes of detecting the presence of a human hand or other body member within a prescribed distance from the spray tip, and thereby disabling the spray gun trigger mechanism.
In spray guns of the type which spray atomized particles under the force of relatively high hydrostatic pressures, there exists the problem that sprayed particles in the immediate vicinity forward of the spray tip travel at velocities sufficient to penetrate human skin. If such particles are paint or other body contaminating material they may cause serious infection or other medical complications which require extensive medical treatment. The seriousness of this skin penetration problem is in some way related to the hydrostatic pressure of the sprayed fluid particles, and it is generally assumed that hydrostatic pressures in excess of 1000 pounds per square inch (psi) are sufficiently high to cause skin penetration. This problem requires that a high degree of care be exercised when operating spray guns above the apparent hydrostatic pressure danger lever to avoid close contact with the spray tip. The region of dangerous proximity to the spray tip is apparently several inches or less, it generally being understood that beyond a range of several inches the individual fluid particles have lost sufficient velocity to be unable to penetrate the skin.
It has been found that the quality of spray coating which may be achieved on any given surface generally improves as the hydrostatic pressure of the sprayed paint is increased. Hydrostatic pressures in the range of 2,000-3,000 psi have been found to produce excellent coating results, and such spraying pressures are commonly used in industry. If the hydrostatic pressure is reduced in order to avoid the foregoing problem the resulting degradation in film quality of the coating material is readily apparent. Therefore, while it is necessary to use extreme care in the handling of a spray gun at such elevated pressures, they are nevertheless used at these pressures in order to achieve the desired quality of film coating.
A number of safety features have been devised in the prior art to permit the use of such spray guns at elevated pressures while at the same time providing operator safeguards. For example, U.S. Pat. No. 3,952,955, issued Apr. 27, 1976, and owned by the same assignee as the present invention, discloses a spray tip guard which is connected to the forward exterior end of the spray tip and presents an obstacle and barrier to anyone attempting to insert fingers or hands near the spray tip. U.S. Pat. No. 3,944,141 issued Mar. 16, 1976, discloses a safety hood apparatus for shielding the region forward of the spray tip, and which may be pivotally moved away from the spray tip for cleaning or replacement purposes, but in so doing disables the trigger actuating mechanism. U.S. Pat. No. 3,913,844 issued Oct. 21, 1975 discloses yet another approach to a trigger disabling mechanism which disables the trigger whenever the spray tip locknut is attempted to be removed.
All of the foregoing safety devices provide mechanical means for protecting the operator against inadvertent contact with the spray tip. Since all of them require some degree of maintenance and care they impose a certain degree of responsibility upon the operator to properly maintain and care for them. Unfortunately, an operator who finds this task disagreeable can find ways to defeat each and every one of the foregoing mechanical safety features. It is therefore a desirable object to provide a safety apparatus which may not be disconnected by operator action or otherwise disabled by operator manipulation, and it is an object of the present invention to fulfill this purpose.